Testing underway of connections between the diaphragm wall and concrete floor slabs at Hung Hom Station in Hong Kong suggest that construction flaws are widespread in the structure.
A preliminary report from the Hong Kong Highways Department shows that over a third of the connections checked failing key criteria and at three locations there was no connection between the rebar and floor slab coupler.
Work to inspect key locations on site started on 10 December and set out to expose connections between the diaphragm wall and concrete platform slabs to settle claims that steel connections had been cut short.
The approach agreed between the government and the MTR Corporation involved exposing up to two full threads on the coupled and proving that the embedded length of the threaded steel bar inside the coupler was at least 40mm.
The Highways Department said that the test locations were randomly selected based on a statistical approach.
MTR is using a mix of physical exposure and ultrasonic testing to prove the embedment. As the allowable measurement tolerance of the test is 3mm, equipment readings below 37mm are regarded as failure in meeting the requirement.
Test results published so far show that 11 of the 32 locations fall below the 37mm embedment criteria, plus three further locations showed no connection.
The Hong Kong Highways Department has not yet commented on the results and testing in ongoing.
MTR is expected to use the as-constructed details of the platforms and quality of works, as well as take into account the technical data provided by the coupler supplier, to conduct a detailed structural analysis to determine if it is necessary to conduct strengthening works.
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Hung Hom in detail
GE’s sister title NCE visited the Hung Hom construction site in 2014 during the diaphragm wall construction for the remodelling work, which is currently under contention.
Located at the southern end of the Shatin to Central Link, the site was previously occupied by a railway freight yard.
The diaphragm walls for the new part of the station were constructed in a 6m high space below the existing station using a specially adapted low clearance diaphragm wall rig.
NCE reported: “In between the rows of structural columns supporting the station above, the rigs are creating more diaphragm walls, this time for the cut and cover tunnels and the new 300m long platform and station box which contain Shatin to Central Link and East Rail Line platforms, that will be relocated from the west side of the station, making it easier for passengers to change lines.
“These walls are between 20m and 50m deep depending on the depth of the bedrock, and the height constraints means that the reinforcement cages can be no more than 4m tall.
“MTR is using some of the freight yard area to set up a soundproofed reinforcement production yard. The factory is currently producing 1,000 threaded rebar cages per day, but this is expected to rise to 1,500 as concrete pours rise to a peak of around 500m3 per day.”
The Hung Hom Station on the Central to Shatin Link connects to an immersed tub tunnel which will take the new rail line across Victoria Harbour and was expected to be completed in 2020.